Snapping turtles do attack ducks, particularly ducklings. You can even find videos with them going after the ducks.Ihave never seen any on our property, but I believe they are around our area.
But I'm not an expert on this.
The wound being under the duck seems to fit this. If it were anything else it would be above or on the up or top or sides not below. Below would imply a pond water predator in the water, I think.